:root {
    --background-color: #2c3e50;
    --line-color: slategray;
}

* {
    margin: 0;
    padding: 0;
}

body {
    width: 100vw;
    height: 100vh;
    background-color: var(--background-color);
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
}

.item {
    position: relative;
    width: 120px;
    height: 40px;
    display: flex;
    justify-content: center;
    align-items: center;
    /* background-color: cadetblue; */
}

.item .line {
    width: 120px;
    height: 3px;
    background-color: var(--line-color);
    animation: line_width_change 1s linear infinite;
}

@keyframes line_width_change {
    0%, 100% {
        width: 100%;
    }
    50% {
        width: 0%;
    }
}

.item::before, .item::after {
    position: absolute;
    content: "";
    width: 30px;
    height: 30px;
    border-radius: 50%;
    background-color: yellow;
}

.item::before {
    background-color: salmon;
    left: -15px;
    animation: circle_move1 2s linear infinite;
    z-index: 100;
}

.item::after {
    background-color: seagreen;
    right: -15px;
    animation: circle_move2 2s linear infinite;
    z-index: 200;
}

@keyframes circle_move1 {
    0% {
        left: -15px;
        width: 30px;
        height: 30px;
        z-index: 100;
    }
    25% {
        width: 25px;
        height: 25px;
    }
    50% {
        left: 105px;
        width: 30px;
        height: 30px;
    }
    75% {
        width: 35px;
        height: 35px;
    }
    100% {
        left: -15px;
        width: 30px;
        height: 30px;
        z-index: 200;
    }
}

@keyframes circle_move2 {
    0% {
        right: -15px;
        width: 30px;
        height: 30px;
        z-index: 200;
    }
    25% {
        width: 35px;
        height: 35px;
    }
    50% {
        right: 105px;
        width: 30px;
        height: 30px;
    }
    75% {
        width: 25px;
        height: 25px;
    }
    100% {
        right: -15px;
        width: 30px;
        height: 30px;
        z-index: 100;
    }
}

.item:nth-child(1) .line, .item:nth-child(1)::before, .item:nth-child(1)::after {
    animation-delay: 0.1s;
}
.item:nth-child(2) .line, .item:nth-child(2)::before, .item:nth-child(2)::after {
    animation-delay: 0.2s;
}
.item:nth-child(3) .line, .item:nth-child(3)::before, .item:nth-child(3)::after {
    animation-delay: 0.3s;
}
.item:nth-child(4) .line, .item:nth-child(4)::before, .item:nth-child(4)::after {
    animation-delay: 0.4s;
}
.item:nth-child(5) .line, .item:nth-child(5)::before, .item:nth-child(5)::after {
    animation-delay: 0.5s;
}
.item:nth-child(6) .line, .item:nth-child(6)::before, .item:nth-child(6)::after {
    animation-delay: 0.6s;
}
.item:nth-child(7) .line, .item:nth-child(7)::before, .item:nth-child(7)::after {
    animation-delay: 0.7s;
}
.item:nth-child(8) .line, .item:nth-child(8)::before, .item:nth-child(8)::after {
    animation-delay: 0.8s;
}
.item:nth-child(9) .line, .item:nth-child(9)::before, .item:nth-child(9)::after {
    animation-delay: 0.9s;
}
.item:nth-child(10) .line, .item:nth-child(10)::before, .item:nth-child(10)::after {
    animation-delay: 1s;
}